1. Field of the Invention
The present invention relates generally to implantable cardiac devices, such as implantable cardioverter defibrillators (ICDs). The present invention more particularly relates to inhibiting voltage delay in a battery of an implantable cardiac device.
2. Background Art
Implantable cardiac devices, such as implantable cardioverter defibrillators (ICDs), are well known in the art. Such devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The primary components of an ID include a monitoring and detection mechanism, a capacitor, a battery, a sensing system for detecting an arrhythmia, and a control system for controlling delivery of a capacitive discharge electrical shock in response to a detected arrhythmia by charging and then discharging the capacitor. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are positioned within the heart for making electrical contact with the muscle tissue of their respective heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired electrical therapy.
Implantable devices, such as ICDs, pose a unique demand on the battery power supply. The battery must be capable of operating at low current drains for long periods of time, and simultaneously be capable of providing high current pulses. The battery must sustain the provision of the high current pulses and must be available whenever a patient requires defibrillation.
Batteries that are used in current ICD""s can develop high internal impedance or equivalent series resistance (ESR) over the lifetime of the ICD. This is undesirable because a battery with an excessively high ESR will ultimately be unable to deliver sufficient current to the circuitry of the ICD leading to a potentially catastrophic result to the patient. Batteries with an ESR that is higher than normal also take longer to charge the capacitor of the ICD, thus potentially delaying the delivery of vital therapy to the patient. Under normal operations, a battery in an ICD should be capable of charging a capacitor in less than about fifteen seconds. Batteries with a higher than normal ESR may take twice as long. Note, many ICD""s are programmed to xe2x80x9ctime outxe2x80x9d after a certain charge time to save energy. For example, an ICD""s may be programmed to stop capacitor charging after 30 seconds. The time interval at the start of a discharge during which the working voltage of a battery cell is below its steady value (which is caused by the higher than normal ESR) is referred to as the xe2x80x9cvoltage delay.xe2x80x9d ESR will also increase when the battery is completely discharged. However, this ESR is not associated with voltage delay which is normally a phenomenon that occurs in the middle of the discharge curve.
Accordingly, it is imperative to limit the development of a high ESR in ICD batteries to thereby inhibit voltage delay. Generally, when a battery reforms a capacitor on a periodic basis, the battery system itself reduces its ESR through the usage. While this technique is effective in reducing battery ESR, it should not be used more often than about once a month because the process is extremely energy-inefficient. For example, each reforming charge of the capacitor may require the withdrawal of 30-40 joules of energy from the battery. If this is performed on a regular schedule to limit ESR development in the battery, it would squander a large amount of battery energy.
While existing ICD batteries have proven effective, it would be desirable to improve the effectiveness and efficiency of ICD batteries by limiting ESR development in ICD batteries such that the batteries are maintained at a predetermined ideal state of voltage and current delivery capacity. In other words, it is desirable to inhibit battery voltage delay in a manner that wastes as little battery energy as possible.
The present invention is directed towards methods and devices for inhibiting voltage delay in a battery of an implantable cardiac stimulation device. These methods and devices of the present invention can also be used to reform a capacitor of the implantable cardiac stimulation device. Such a capacitor (which is typically implemented as two capacitors in series for increased voltage handling capability) is charged using the battery. The charge on the capacitor can then be used to shock the heart of the patient within which the stimulation device is implanted. If the capacitor was charged for a reason other than for delivering a shock (e.g., for inhibiting battery voltage delay and/or reforming the capacitor), then the charge on the capacitor can be allowed to slowly dissipate or discharged using a dump circuit.
According to an embodiment of the present invention, the capacitor begins to be charged at a time determined based on a comparison between a time since a last charge of the capacitor and a threshold time between charges. For example, the capacitor may begin to be charged when the time since the last charge of the capacitor equals the threshold time between charges. A charge on the capacitor is then measured at a predetermined time since the capacitor began charging. The capacitor continues to be charged until a threshold charging time since the capacitor began charging is reached, at which point charging stops. The threshold charging time is then adjusted based on the measured charge on the capacitor. This threshold charging time can be adjusted prior to or after the stopping of the charging of the capacitor.
In an alternative embodiment, rather than adjusting the threshold charging time, the threshold time between charges is adjusted based on the measured charge (i.e., voltage) on the capacitor. In another embodiment, both the threshold charging time and the threshold time between charges are adjusted.